Control system for mobile air conditioning apparatus

ABSTRACT

A control system for mobile air conditioning systems for controlling the rate at which the system compressor or clutch is cycled between operational and decreased refrigerant flow, by selectively controlling the response time of certain system control signals to flow demand changes including de-icing control, from interactions between de-icing control and flow control valves, as well as system overall control. Upon the detection of an icing condition, the response of the control system is delayed to a degree to control the rate at which the capacity of the compressor can be cycled with in design limits. The delay feature allows the differential used in differentiating between icing and non-icing conditions to be significantly reduced over that required in the prior art and thereby improving the performance of the air conditioning system. Icing conditions, or the likelihood thereof, can be determined by sensing the condition of the refrigerant flow from the evaporator In addition, the history of compressor cycling can be stored and used to prevent excessive cycling for any reason.

FIELD OF THE INVENTION

This invention pertains to mobile air conditioning systems controls forcontrolling compressor cycling in general and more particularly while inthe evaporator de-icing mode, for increased system performance.

BACKGROUND OF THE INVENTION

The design criterion of mobile air conditioning systems requires thatthe air conditioning systems be designed to operate as efficiently aspossible over a wide variety of environmental and operating conditionsin a manner so as not detrimentally impact the capability of theequipment to properly perform, particularly the compressor or itsclutch, as environmental and operation conditions change. One of thedemands on operation of the compressor or its clutch is created bymobile air conditioning systems evaporator de-icing control. Evaporatoricing is caused when the evaporator temperature is dropped to a level,that when air is blown through the evaporator, the humidity in the aircauses ice crystals to form on the fins and tubing within theevaporator. If this continues for an extended period of time, the ice soformed will impede the flow of air through the evaporator, reducing thecooling capacity of the air conditioning system, and in the extreme, ifsufficiently entirely iced over, essentially cutting off most of the airflow, thereby effectively incapacitating the air conditioning system.

The mobile air conditioning systems of the prior art, such as thoseincluded in automobiles, trucks, busses, etc., presently include atemperature or pressure sensor at the output of the evaporator to detectwhen the evaporator is operating at a condition that is likely to causeicing. When the icing condition is sensed, the sensor reduces thecompressor refrigerant flow capacity by operating a clutch relay, orswitch, to deactivate the compressor clutch and thereby stop refrigerantflow until the sensor detects the selected non-icing condition. Suchmobile air conditioning systems are usually driven by the vehicle enginethrough an electrically operated clutch. The clutch is initiallyactivated when the air conditioning system is turned on, and then turnedon and off by the vehicle air conditioning system temperature controlsystem to maintain the vehicle at the desired temperature. In the latervehicle models, the evaporator icing sensor is connected to a vehiclecomputer that controls the activation and deactivation of the clutch.

It has been found that the higher the humidity of the atmosphere, morelikely that icing can occur in the evaporator. Hence, mobile airconditioning systems are more susceptible to icing in the more humid,locations, which can be considered as the "worse case" for designpurposes. As in most mechanical devices, mobile air conditioning systemclutches have design limits as to the number of times the clutch can becycled on and off over a period of time with out causing deteriorationor break down. In the case of automobile air conditioning systems, it isrecommended that the compressor clutch should cycle on and off no morethan six times a minute. Hence, it is important that the design ofmobile air conditioning systems operates efficiently over a wide rangeof environmental and operating conditions, without evaporator icing, andwithout causing excessive cycling of the compressor clutch.

The present approach of the prior art to maintain a balance betweenevaporator de-icing control and compressor cycle rate is to include alarge differential in the icing detector sensor between selectednon-icing and icing conditions. The large sensor differential wasselected was that needed for worse case conditions (i.e. expected worsecase humidity, temperature, etc. conditions). The sensor was set so thatthe selected large differential, along with the response time of the airconditioning system, assures that de-icing system clutch cycling demandsare within recommended cycling design limits at worse case conditions.For example, in some models of automobiles the icing condition isselected at 25 psi., while the non-icing condition is selected at 45psi. (system reset condition), a significant differential of 20 psi.Although this arrangement was found to be a satisfactory to preventevaporator icing in mobile air conditioning systems for worse caseconditions, this large differential penalizes the performance of the airconditioning system in other than worse case conditions by keeping thecompressor in the off condition over a wider range of the temperaturesthan needed. For example, if the compressor is shut off at the icingcondition of 25 psi., the entire air conditioning system remains shutoff until the preset differential high limit of 45 psi. is reached, asignificant dead time required for the worst case operation. Since suchlarge differential was selected for the worse case condition, the samelarge differential exists for all other operating conditions, despitethe fact that the large differential is not needed and therebyunnecessarily detrimentally impacts the performance of the system underthe large majority of operating conditions by keeping the system shutdown longer than needed. It would therefor be advantageous if thedifferential detection range between the icing and non-icing conditionscould be reduced to improve the overall performance of the airconditioning system while still not exceeding the recommended clutchcycling rate.

Further, the use of a pressure sensing compressor clutch deactivatingsystem of the prior art was found to be undesirable in mobile airconditioning systems using a temperature sensitive refrigerant controlvalve (that controls the refrigerant flow through the system). Suchtemperature sensitive control valves are connected in the output line ofthe evaporator to detect the evaporator output temperature and controlthe system refrigerant flow as a function of evaporator outputsuperheat. The temperature sensing mechanism of the control valve isinherently slow in its reaction both in its refrigerant flow restrictingand increasing flow modes. With a pressure sensing evaporator icingcontrol included in such system, it was found that when the controlvalve cuts back refrigerant flow and an icing condition issimultaneously sensed, the combination of the response times of theoperation of the valve and the icing control system interact to causethe compressor clutch to rapidly cycle off and on several times beforethe system is stabilized. This clutch cycling not only is detrimental tothe life of the clutch but was found to be annoying to the vehicleoperator. It would therefor be advantageous to be able to include anevaporator deicing control system in a mobile air conditioning systemsusing a temperature controlled control valves that would be operable insuch systems and still avoid the repetitive cycling of the compressorclutch due to valve and icing system interaction.

It is therefor an object of this invention to provide a new and improvedcompressor flow control arrangement for controlling compressor or clutchcycling in mobile air conditioning systems that provides for the use ofthe evaporator de-icing control arrangement that has a significantlyreduced range between icing and non-icing determinations to therebyimprove the overall system performance while maintaining compressorclutch cycling within design limits.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the invention, the response times in a mobile airconditioning system are changed for controlling the rate at which thesystem responds to refrigerant flow demands due to evaporator deicingcontrol, so as to interact with refrigerant flow control valves andde-icing control, as well as system control, in a manner so as tocontrol the rate at which the compressor refrigerant flow capacity canbe cycled on and off for icing control, while increasing overall systemperformance by allowing for a reduced differentials between icing andnon-icing determinations. The icing and non-icing determinations can bemade by sensors that are connected adjacent to the evaporator to senserefrigerant temperature or pressure.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic diagram of an orifice type of mobile airconditioning system including the evaporator icing control system of theinvention.

FIG. 2 is a schematic diagram of a temperature sensing control valvetype of mobile air conditioning system including the evaporator icingcontrol system of the invention.

FIG. 3 is a first embodiment of an icing detection and control circuitfor introducing a time delay, in accordance with the invention, into thedeactivation of the clutch in response to sensor evaporator icingsignals, wherein the sensor is connected directly in the clutch circuitand a time delay relay contacts are connected in parallel to the sensor.

FIG. 4 is a second embodiment of an icing detection and control circuitfor introducing a time delay, in accordance with the invention, into thedeactivation of the clutch in response to the sensor evaporator icingsignals, wherein the sensor is connected series with a clutch actuatingrelay that includes a time delay relay by pass circuit.

FIG. 5 includes a schematic diagram of another embodiment of an orificetype mobile air conditioning system having a computer to control the airconditioning system and including the evaporator icing control system ofthe invention.

FIG. 6 includes a flow diagram for modifying the software in thecomputer of FIG. 5 to include the delay of the icing control system ofthe invention.

FIG. 7 includes a flow diagram for modifying the software in thecomputer of FIG. 5 to include an overall clutch cycle control feature ofthe invention.

FIG. 8 includes a schematic diagram of another embodiment of atemperature sensing valve controlled type mobile air conditioning systemhaving a computer to control the air conditioning system and includingthe evaporator icing control system of the invention.

FIG. 9 includes a flow diagram for modifying the software in a computerof FIG. 8 to include a valve motion detection in the icing controlsystem of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The mobile air conditioning system of FIG. 1 includes a compressor 10that is coupled to be driven by the vehicle engine (not shown) thatpowers both the vehicle and the mobile air conditioning system. In thecase of an automobile air conditioning systems the compressor is usuallydriven by a belt coupling between the engine and the compressor pulley12. The compressor 10 is driven via a clutch 14 when activated. Thespeed at which the compressor 10 is rotated is a function of the speedof rotation of the vehicle engine. Hence the higher the speed orrotation of the engine, the higher the speed of rotation of thecompressor, and therefor the higher the capacity of the compressor topump refrigerant. The compressor 10 is turned on and off via the clutch14 by the air conditioning system control system 30 as the need for airconditioning arises.

The direction of refrigerant flow through the system is illustrated bythe arrows 24. A compressed high pressure gaseous refrigerant flows fromthe compressor 10 through a condenser 16. The purpose of the condenser16 is to reject heat from the air conditioning system, while at the sametime condenses the high pressure gaseous refrigerant into high pressureliquid refrigerant. Air flow through the condenser 16 absorbs heat fromthe refrigerant. In mobile air conditioning systems, the flow of airthrough the condenser 16 is variable and is controlled by thecombination of the speed at which the vehicle is traveling, the speed atwhich the engine fan is rotating , and the speed at which a condenserfan (not shown) is rotating, wherein the greater the air flowtherethrough the greater the heat rejection, and visa versa.

The high pressure liquid refrigerant flows through an orifice 20, whichis of a fixed size and restricts refrigerant flow through the airconditioning system and particularly the amount of refrigerant flowthrough the evaporator 18. Ideally a low temperature, all liquidrefrigerant flow should enter the evaporator 18. The evaporator output23 should be all vapor at the saturation temperature (boiling point). Itis the boiling of the refrigerant within the evaporator 18 that causesheat absorption and provides the cooling effect of the evaporator 18. Ablower 21 blows air through the evaporator 18 cooling the air as itpassed through the evaporator providing for vehicle cooling. The speedcontrol (not shown) for the blower 21 can be the existing multi-speed ofvariable speed types. As previously mentioned above, in certainenvironmental conditions that the vehicles are exposed to, particularlyin high humidity, the evaporator 18 has a greater tendency to ice over.In addition, the high humidity along with vehicle speed and engine speedfurther increases the tendency for icing. This icing of the evaporator18 may seriously impact the efficient flow of air through the evaporatorand thereby detrimentally impact the system performance.

The refrigerant flows from the evaporator 18 through an accumulator 22back to the compressor 10. If liquid refrigerant flows from theevaporator 18, the accumulator 22 will accumulate the liquid. If noliquid refrigerant flows from the evaporator 18, the accumulator 22bleeds liquid refrigerant into the system. In effect the accumulator 22controls the amount of active refrigerant charge in the system.

In accordance with the invention, an evaporator icing detection andcontrol circuit 26 includes a sensor connected adjacent the output ofthe evaporator 18 to sense the operation of the evaporator 18 todetermine if the evaporator 18 is likely to be in, or about to be, in anicing condition. The sensor of FIG. 1 is illustrated as located in therefrigerant flow line of the accumulator 22, however it can be locatedin the flow line 23 itself. The sensor can determine if the icingconditions are likely to be present through either, temperature orpressure measurements, although pressure is preferred due its inherentquicker response time by avoiding the thermal lag usually involved intemperature sensing. If a temperature sensor is to be used, the sensorcould be alternatively placed within the evaporator 18.

As mentioned above, the mechanical limits of the clutch 14 are such thatthe clutch 14, or compressor, should only be cycled within its designlimits or else be subject to deterioration or break down due tomechanical or electrical failures. It was because of these design limitsthat the icing sensing system of the prior art included an evaporatorsensor with an undesirable selected large differential gap between icingand non-icing signal levels for worst case cycling conditions. Thislarge differential was selected to create a limit on the switchingcycling rate that could be applied by the icing control system to theclutch 14 at worst case cycling conditions. Since the large sensordifferential of the prior art was selected for the worse case cyclingconditions, the same undesirable large differential limitations areapplied to all other operating conditions (non-worse case) and thereforresults in an associated overall loss in system performance where thelarge differential is not needed.

The limitations on system performance introduced by the undesirablelarge sensor differential of the prior art icing control systems arereduced, in accordance with the invention, by controlling the responsetime by which the clutch can respond to de-icing signals, by theintroduction of off and/or on delays, control switching sequences etc.This introduction of controlled response times allows for a significantreduction in icing sensor differentials between icing and non-icingdetection levels that can be used without exceeding clutch or compressorcycle limits. Hence, with a reduced icing and non-icing differential,the mobile air conditioning system will operate at improved performancefor all operating conditions, while through the innovative use ofcontrolled response times through delays or switching sequencesexcessive clutch cycling is avoided.

The icing detection and control circuit 26 of FIG. 1 is connected inseries between the air conditioning system control circuit 30 and thecompressor clutch 14 circuit. The icing detection and control circuit 26can include a sensor with a built in mechanical delay, or the delay canbe created electrically as illustrated in FIGS. 3 and 4, orelectronically, or by computer programming as illustrated in FIGS. 5 and6. Whenever the sensor detects an icing condition, a delay period iscreated during which time the compressor clutch 14 can not bedeactivated, or activated, by the icing sensor.

In accordance with one embodiment, after the delay period has run, theicing detection and control circuit 26 opens the connection to theclutch and the clutch is deactivated and the flow of refrigerant is cutoff until the non-icing condition is detected. When the non-icingcondition situation is detected by the sensor and after the delay hasrun, the circuit to the clutch 14 is again closed via the icingdetection and control circuit 26 and the refrigerant flow is reactivatedby the compressor 10 under the control of the air conditioning controlcircuit 30. In accordance with another embodiment the clutch could beimmediately deactivated with its reactivation delayed. Hence, it now canbe seen, that according to the invention, the limit for cycling theclutch can be controlled by a delay and now a smaller sensordifferential can now be used to distinguish between the icing andnon-icing modes. By reducing the icing sensor differential, thedesignated lower level of non-icing condition is detected sooner. Thisin turn provides for a shorter period of system operation down time (ashorter period of time during which the refrigerant flow is interrupted)and therefor a correspondingly better the system performance. Therefor,in accordance with the invention, it can be seen the penalty thataccompanied the undesirable large sensor differential of the prior art(set by worst case driving conditions) can now be significantly reducedwhile still safeguarding the cyclic operation of the clutch 14 by theuse of a delaying action and/or switching sequences.

For the purpose of simplifying the explanation of the invention, inFIGS. 1, 2, 5 and 8 the same elements in each of these Figures will havethe same references numerals. The mobile air conditioning system of FIG.2 includes a temperature sensing control valve 32 instead of the orifice20. The valve 32 detects the temperature of the refrigerant flow fromthe evaporator 18 to control the refrigerant flow through the airconditioning system. The evaporator icing detection and control circuit26 is located in the flow line 37 before the valve 32, but could belocated after the valve 32, or if a temperature sensor is used, withinthe evaporator 18. The icing sensor circuit 26 functions in the samemanner as described with regard to FIG. 1.

FIG. 3 includes a first embodiment the an icing detection and controlcircuit 26 for use in FIGS. 1 and 2. A set of normally closed contacts41 of the icing sensor 40 are connected in series between the airconditioning system control 30 and the clutch 44 coil 45 to ground 43. Acoil 47 of a delay relay 46 is connected between the contacts 41 and thecoil 45 and ground. The normally open contacts 48 of the time delayrelay 46 are connected across the sensor contacts 41. The delay relay 46is of the type that continues in the activated condition for a presetperiod of time after being de-energized. In the non-icing mode, thesensor contacts 41 are closed allowing the clutch coil 45 to beenergized by the air conditioning control 30 and the time delay relaycoil 47 is also energized to close contacts 48. When the sensor 40detects an icing condition the contacts 41 open. However, the contacts48 of the time delay relay 46 remain closed and by pass the sensorcontacts 41 for a preset period of the time delay. When the delay periodof time has expired, the contacts 48 open and the clutch coil 45 isde-energized to shut off the compressor. The period of delay timeselected for the relay 46 is of a duration to keep the sensor contacts41 by passed by the contacts 48 so that the clutch 14 can not be cycledby the sensor contacts 41 beyond its design limits. With the clutch 14disengaged, the flow of refrigerant ceases allowing the evaporator towarm up. When the sensor 40 detects the selected non-icing condition,the contacts 41 are closed to energize the clutch coil 45 and restartthe flow of refrigerant, while also simultaneously energizing the timedelay relay 46. If the sensor contacts 41 close before the delay time ofthe relay 46 has expired, the delay relay 46 is re-energized and theclutch 14 remains continuously energized for an uninterrupted flow ofrefrigerant.

FIG. 4 includes second embodiment of an icing detection and controlcircuit 26 for use with FIGS. 1 and 2. A power relay 54 is connected inseries with the sensor 50 normally closed contacts 51. The relay 54 coil56 is actuated and de-actuated in response to the closing and opening ofthe sensor contacts 51. The power relay normally open contacts 55 areconnected between the system control 52 and the clutch 64 coil 63. Thecircuit of FIG. 4 has the advantage of removing the sensor contacts 51out of the high current clutch coil 64 circuit. In FIG. 4 the time delayrelay 58 coil 60 is energized whenever the relay 54 is energized and thenormally open contacts 62 are connected in parallel to the power relaycontacts 55. Whenever the contacts 51 are opened due to the detection ofan icing condition, the relay 54 is opened , but the contacts 62continue to by pass the contacts 55 for the time delay of the time delayrelay 58 preventing the de-energization of the clutch coil 63 during thetime delay period. When the sensor 50 detects the preset non-icingcondition, the contacts 51 are closed to energize the relay 58, thedelay relay 46 and the clutch coil 60.

Although the circuits of FIGS. 3 and 5 provide an arrangement whereinthere is a time delay introduced before the clutch can be deactivated,it should be understood that the circuits can be modified so that theclutch coil can be immediately deactivated and a time delay introducedbefore the clutch could be reactivated.

Although the electrical control circuits of FIGS. 3 and 4 disclose theconcept of inserting delays into the air conditioning system controlwith the use of sensor switch contacts and relays, it should beunderstood that the icing detection and control circuit 26 could as wellbe embodied with the use of well known solid state detectors, delaycircuits and switching devices. With the added use of computers inmobile air conditioning systems, the vehicle computer that controls theoverall operation of the air conditioning system, including theevaporator icing control for the system, can be used, in accordance withthe invention, to improve the performance of the system by providing thedelaying action for limiting the rate at which the compressor clutch 14can be cycled on and off.

As illustrated in FIG. 5, the vehicle computer 74 of the prior artreceives signals from the air conditioning control 30 to control theoperation of the air conditioning system, and receives icing conditionsignals from an evaporator icing sensor 70. In accordance to theinvention, the portion of the prior art computer software in thecomputer 74 pertaining to the energization of the clutch 14, via theclutch control circuit 72, is modified to insert time response controlfor limiting the cycle rate at which the clutch 14 can be turned on andoff. For example, instead of responding to the icing signal from sensor70 to immediately de-energize the clutch 14 (as done in the prior art),the computer 74 is programmed to introduce a delay time to prevent theclutch 14 from being de-activated until the delay period has run, oralternatively to program the computer 74 to immediately de-energize theclutch and to delay the re-activation of the clutch.

The flow diagram of FIG. 6 includes a modification of the computer 74software to include the time response control concept of the invention.The first step 76 of the process determines if the vehicle engine isrunning, while the second step 77 determines if the air conditioningsystem is turned on. Step 78 determines if there is a request for airconditioning from step 84 (which compares the air conditioningtemperature setting with the internal vehicle temperature). If there isa request for air conditioning, the step 78 enables the clutch engagestep 79 to enable the clutch relay 80 to engage the clutch 81. Howeverif an icing condition is determined by step 82, instead of its outputbeing directly applied to disable the clutch (as done in the prior art,a delay step 83) is included between steps 79 and 82. The delay can bean off delay as in FIGS. 3 and 4, or can be an on delay wherein the step79 is immediately disabled and a delay is inserted in engaging theclutch. The delay is that needed to keep the clutch from cycling beyonddesignated limits.

The flow diagram of FIG. 7 includes a modification of the flow diagramof FIG. 6 that includes an overall clutch cycle control feature. Thestep 96 continually counts the rate at which the clutch is cycled ingeneral, and the cycle rate is compared with a preset limit by the step97. If the preset limit is exceeded, the step 97 applies a limitexceeded indication to the disengage step 98 and to the delay step 99.The delay step 99 can be an off delay or an on delay. If an off delay isused, then the arrangement is such that the disengage step 98 applies adisengage designation to the clutch engage step 79 after the delay ofstep 99 has run. If an on delay is used then the step 98 applies thedisengage immediately upon receipt of the limit exceeded status and canbe re-engaged after the delay of step 99 has run. The duration of thedelay is that needed to keep the clutch cycle rate within design limits.

In accordance with FIG. 8, the vehicle computer 74 has been programmedto function in the same manner in response to icing signals from sensor70 as in FIG. 5. However, the computer 74 is also programmed to beresponsive to signals from a motion detector 75 to detect the operationof the temperature sensitive control valve 32 to prevent the repetitivecycling of the clutch 14 due to the interaction of icing control andvalve flow control as mentioned above. The motion detector 75 is coupledto the temperature sensitive control valve 32 to detect valve motions ofmagnitudes that might cause the interactive cycling. The program of thecomputer 74 is modified to temporarily inhibit the computer 74 fromacting on the icing signals from the sensor 70 during the motion of thevalve 32 that might have other wise interacted with the icing controlcaused the repetitive clutch cycling.

The flow diagram of FIG. 9 includes a modification of the software ofthe computer 74 of FIGS. 6 and 8 so as to reduce the cyclic interactionbetween the temperature controlled valve 32 and the icing sensor 70. Themotion magnitude step 106 determines when the motion temperaturecontrolled valve 32 exceeds predetermined limits and enables the motionsignal generator step 107 to apply a motion designation to theinteraction step 108. The interaction step 108 also receives an icingdesignation from the icing condition step 82. When both the icing andmotion designations are present, the output of step 108 enables theinhibit step 109 to inhibit the application of the icing conditioncontrol until the motion of the temperature controlled valve motion iswithin limits to minimize the interaction mentioned above.

As described above, in accordance with the invention, a delay isinserted in the clutch control circuit for controlling the clutch cyclerate instead of the inefficient use of the large sensor differential ofthe prior art. The delay can be set for 10 seconds so as not to exceedthe design recommendation of a maximum of 6 clutch cycles per minute.Under normal conditions (non-worse case conditions) with the likelihoodof excessive clutch cycling decreasing, the sensor icing--non icingdifferential can be significantly decreased, which in turn reducessystem dead time resulting in a corresponding improvement in systemperformance.

In more sophisticated systems using computers by which the operation ofthe clutch is controlled, the invention can be simply installed byreplacing the prior art icing sensor with a sensor with a smallerdifferential and by programming the delay into the computer. Thecomputer can have the added advantage of accumulating data as to theperiodic and cyclic operation of the clutch, and if history allows, onlyinsert the delay when required.

What is claimed:
 1. An evaporator de-icing control system for a mobileair conditioning system comprising:means for determining when theevaporator is likely to be in an icing condition and when the evaporatoris in a non-icing condition; refrigerant flow control means for reducingthe level of refrigerant flow in the system when an icing condition isdetermined and for resetting the refrigerant flow to the normal operatelevel when an non icing condition is determined, and means forinitiating a delay in the time response by which the refrigerant flowcontrol means reacts to the at the occurrence determination of at leastone of the icing and non-icing conditions.
 2. A control system asdefined in claim 1 wherein:the determining means is a sensor locatedadjacent to the evaporator so as to determine whether the evaporator isin an icing condition or in a non-icing condition.
 3. A control systemas defined in claim 2 wherein:the sensor is a refrigerant pressuresensor mounted to sense pressure adjacent the output of the evaporator.4. A control system as defined in claim 2 wherein:the sensor is atemperature sensor mounted to sense the evaporator temperature.
 5. Acontrol system as defined in claim 1 wherein:the refrigerant controlmeans is a compressor clutch wherein a clutch is disengaged and engagedby the determining means.
 6. A control system as defined in claim 5wherein:the means for initiating a delay includes a delay means thatinserts a time delay in the time response of the refrigeration controlmeans wherein at least one of the response time of the disengagement andthe subsequent engagement of the clutch is delayed.
 7. A control systemas defined in claim 2 wherein:the refrigerant control means includes acomputer connected to monitor the determinations of the sensor.
 8. Acontrol system as defined in claim 7 wherein:the refrigerant controlmeans is a compressor clutch wherein the clutch is disengaged andengaged by the computer.
 9. In a mobile air conditioning systemincluding an electrical clutch driven compressor for activation by anair conditioning system control system, a condenser, a refrigerant flowcontroller and an evaporator, an evaporator de-icing control systemcomprising:an evaporator sensor for providing an icing indication whenthe evaporator is likely to be in an icing condition and a non-icingindication when the evaporator is in a non-icing condition; circuitmeans connecting the sensor to the air conditioning system controlsystem for enabling the clutch to be activated in response to thenon-icing indication and for deactivating the clutch in response to anicing indication, and means included in the circuit means for responsiveto the determination of one of the icing and de-icing conditions forcreating a predetermined delay in at least one of the response times ofthe deactivation of the clutch in response to an icing indication andthe reactivation of the clutch upon receipt of an non-icing indication.10. An evaporator icing control system of claim 9 wherein:the evaporatorsensor is a refrigerant pressure sensor connected to the airconditioning system adjacent to the output of the evaporator.
 11. Anevaporator icing control system of claim 9 wherein:the evaporator sensoris a temperature sensor located to sense the temperature of theevaporator.
 12. In a vehicular air conditioning system including acompressor, a condenser, a refrigerant flow control mechanism, anevaporator, a compressor clutch having a designated limit on the rate atwhich the clutch is to be cycled between actuated and de-actuatedconditions, a control system for controlling system refrigerant flow byactuating the clutch to enable the compressor to pump refrigerant and tode-activate the clutch to disable the compressor from pumpingrefrigerant, and an evaporator deicing arrangement for providing anicing signal when a determination is made that the evaporator is likelyin an icing condition and a non-icing signal when a preset determinationis made that the evaporator is in a non-icing condition wherein the sizeof differential between icing and non-icing conditions determinations iscontrolled by the selected non-icing determination, and wherein thesensor arrangement interacts with the control system to enable thecontrol system to de-actuate the clutch in response to icing signals andto enable the control system to re-actuate the clutch in response tonon-icing signals, the improvement comprising means responsive to thereceipt of at least one of the icing and non-icing signals from thesensor arrangement for introducing a delay by which the control systemcan respond to deactivate or subsequently re-activate the clutch inresponse to the at least one of the icing and non-icing signals tothereby allow the non-icing determination be set in a direction for areduction in the differential between icing and non-icing determinationsfor improved air conditioning system performance, while controlling therate at which the clutch can be actuated and re-actuated by the icingand non-icing signals by the delayed response of the control system.